1. Field of the Invention
This invention relates generally to pistons for internal combustion engines, including insulated pistons for diesel engines, and methods of manufacturing the same.
2. Related Art
Modern heavy duty diesel engines are being pushed towards increased efficiency under emissions and fuel economy legislation. To achieve greater efficiency, the engines must run hotter and at higher peak pressures. Thermal losses through the combustion chamber become problematic under these increased demands. Typically, about 4% to 6% of available fuel energy is lost as heat through the piston into the cooling system. One way to improve engine efficiency is to extract energy from hot combustion gases by turbo-compounding. For example, about 4% to 5% of fuel energy can be extracted from the hot exhaust gases by turbo-compounding.
Another way to improve engine efficiency includes reducing heat losses to the cooling system by insulating the crown of the piston. Insulating layers, including ceramic materials, are one way of insulating the piston. One option includes applying a metal bonding layer to the metal body portion of the piston followed by a ceramic layer. However, the layers are discrete and the ceramic is by its nature porous. Thus, combustion gases can pass through the ceramic and start to oxidize the metal bonding layer at the ceramic/bonding layer interface, causing a weak boundary layer to form and potential failure of the coating over time. In addition, mismatches in thermal expansion coefficients between adjacent layers, and the brittle nature of ceramics, create the risk for delamination and spalling.
Another example is a thermally sprayed coating formed of yttria stabilized zirconia. This material, when used alone, can suffer destabilization through thermal effects and chemical attack in diesel combustion engines. It has also been found that thick ceramic coatings, such as those greater than 500 microns, for example 1 mm, are prone to cracking and failure.
Although more than 40 years of thermal coating development for pistons is documented in literature, there is no known product that is both successful and cost effective to date. It has also been found that typical aerospace coatings used for jet turbines are not suitable for engine pistons because of raw material and deposition costs associated with the highly cyclical nature of the thermal stresses imposed.